An Apertif to the Next Radio Astronomy Entrée

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To aid in the digestion of a new era in radio astronomy, a new technique for improving the is unfolding at the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands. By adding a plate of detectors to the focal plane of just one of the 14 radio antennas at the WSRT, astronomers at the Netherlands Institute for Radio Astronomy (ASTRON) have been able to image two pulsars separated by over 3.5 degrees of arc, which is about 7 times the size of the full Moon as seen from Earth.

The new project – called Apertif – uses an array of detectors in the focal plane of the radio telescope. This ‘phased array feed’ – made of 121 separate detectors – increases the field of view of the radio telescope by over 30 times. In doing so, astronomers are able to see a larger portion of the sky in the radio spectrum. Why is this important? Well, in keeping with our food course analogy, imagine trying to eat a bowl of soup with a thimble – you can only get a small portion of the soup into your mouth at a time. Then imagine trying to eat it with a ladle.
This same analogy of surveying and observing the sky for radio sources holds true. Dr. Tom Oosterloo, the Principle Investigator of the Apertif project, explains the meat of the new technique:

“The phased array feed consists of 121 small antennas, closely packed together. This matrix covers about 1 square meter. Each WSRT will have such a antenna matrix in its focus. This matrix fully samples the radiation field in the focal plane. By combining the signals of all 121 elements, a ‘compound beams'[sic] can be formed which can be steered to be pointing at any location inside a region of 3×3 degrees on the sky. By combining the signals of all 121 elements, the response of the telescope can be optimised, i.e. all optical distortions can be removed (because the radiation field is fully measured). This process is done in parallel 37 times, i.e. 37 compound beams are formed. Each compound beam basically functions as a separate telescope. If we do this in all WSRT dishes, we have 37 WSRTs in parallel. By steering all the beams to different locations within the 3×3 degree region, we can observe this region entirely.”

In other words, traditional radio telescopes use only a single detector in the focal plane of the telescope (where all of the radiation is focused by the telescope). The new detectors are somewhat like the CCD chip in your camera, or those in use in modern optical telescopes like Hubble. Each separate detector in the array receives data, and by combining the data into a composite image a high-quality image can be captured.

The new array will also widen the field of view of the radio telescope, which allowed for this most recent observation of widely separated pulsars in the sky, a milestone test for the project. As an added bonus, the new detector will increase the efficiency of the “aperture” to around 75%, up from 55% with the traditional antennas.

Dr. Oosterloo explained, “The aperture efficiency is higher because we have much more control over the radiation field in the focal plane. With the classic single antenna systems (as in the old WSRT or as in the eVLA), one measures the radiation field in a single point only. By measuring the radiation field over the entire focal plane, and by cleverly combining the signals of all elements, optical distortion effects can be minimised and a larger fraction of the incoming radiation can be used to image the sky.”This image illustrates the larger field of view afforded by the new instrument. Image Credit: ASTRON

For now, there is only one of the 14 radio antennas equipped with Apertif. Dr. Joeri Van Leeuwen, a researcher at ASTRON, said in an email interview that in 2011, 12 of the antennas will be outfitted with the new detector array.

Sky surveys have been a boon for astronomers in recent years. By taking enormous amounts of data and making it available to the scientific community, astronomers have been able to make many more discoveries than they would have been able to by applying for time on disparate instruments.

Though there are some sky surveys in the radio spectrum that have been completed so far – the VLA FIRST Survey being the most prominent – the field has a long way to go. Apertif is the first step in the direction of surveying the whole sky in the radio spectrum with great detail, and many discoveries are expected to be made by using the new technique.

Apertif is expected to discover over 1,000 pulsars, based on current modeling of the Galactic pulsar population. It will also be a useful tool in studying neutral hydrogen in the Universe on large scales.

Dr. Oosterloo et. al. wrote in a paper published on Arxiv in July, 2010, “One of the main scientific applications of wide-field radio telescopes operating at GHz frequencies is to observe large volumes of space in order to make an inventory of the neutral hydrogen in the Universe. With such information, the properties of the neutral hydrogen in galaxies as function of mass, type and environment can be studied in great detail, and, importantly, for the first time the evolution of these properties with redshift can be addressed.”

Adding the radio spectrum to the visible and infrared sky surveys would help to fine-tune current theories about the Universe, as well as make new discoveries. The more eyes on the sky we have in different spectra, the better.

Though Apertif is the first such detector in use, there are plans to update other radio telescopes with the technology. Dr. Oosterloo said of other such projects, “Phased array feeds are also being built by ASKAP, the Australia SKA Pathfinder. This is an instrument of similar characteristics as Apertif. It is our main competitor, although we also collaborate on many things. I am also aware of a prototype being tested at Arecibo currently. In Canada, DRAO [Dominion Radio Astrophysical Observatory] is doing work on phased array feed development. However, only Apertif and ASKAP will construct an actual radio telescope with working phased array feeds in the short term.”

On November 22nd and 23rd, a science coordination meeting was held about the Apertif project in Dwingeloo, Drenthe, Netherlands. Dr. Oosterloo said that the meeting was attended by 40 astronomers, from Europe, the US, Australia and South Africa to discuss the future of the project, and that there has been much interest in the potential of the technique.